[PDF] - Sunlight Sunlight Yes Yes Y Y A. A. No No B. B. Sunlight PDF Document

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Sunlight 1

Sunlight Sunlight

Sunlight 2

Introductory Question Introductory Question

When you look up at the sky during the day, is

When you look up at the sky during the day, is the light from distant stars reaching your eyes? the light from distant stars reaching your eyes? Y Y

A. A.

Yes Yes

B. B.

No No

Sunlight 3

Observations about Sunlight Observations about Sunlight

Sunlight appears whiter than most light

Sunlight appears whiter than most light

Sunlight makes the sky appear blue

Sunlight makes the sky appear blue

Sunlight becomes redder at sunrise and sunset

Sunlight becomes redder at sunrise and sunset

It reflects from many surfaces, even nonmetals

It bends and separates into colors in materials

Sunlight a key source of renewable energy

Sunlight 4

Reading Question 14.1a Reading Question 14.1a

The sky is blue because

A.

A. blue light scatters off air better than red light

blue light scatters off air better than red light

B.

B. air glows blue when exposed to sunlight

air glows blue when exposed to sunlight

C.

C. sunlight reflected from the earth surface is blue

sunlight reflected from the earth surface is blue

D.

D. sunlight contains more blue light than red light

sunlight contains more blue light than red light

Sunlight 5

Reading Question 14.1b Reading Question 14.1b

Light partly reflects from transparent objects

Light partly reflects from transparent objects because light because light h d i l bj h d i l bj

A.

A. changes speed as it enters or leaves objects.

changes speed as it enters or leaves objects.

B.

B. can’t travel well through transparent objects.

can’t travel well through transparent objects.

C.

C. is unstable inside transparent objects.

is unstable inside transparent objects.

D.

D. has no magnetic field in nonmagnetic objects.

has no magnetic field in nonmagnetic objects.

Sunlight 6

5 Questions about Sunlight 5 Questions about Sunlight

Why does sunlight appear white?

Why does the sky appear blue?

How does a rainbow break sunlight into colors?

Why are soap bubbles and oil films so colorful?

Why do polarizing sunglasses reduce glare?

SLIDE 2

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Sunlight 7

Question 1 Question 1

Why does sunlight appear white?

Why does sunlight appear white?

Sunlight 8

Light Light

Light is

Light is a class of electromagnetic waves a class of electromagnetic waves

Long

Long-

wavelength EM waves: Radio & Microwave

wavelength EM waves: Radio & Microwave

Medium

Medium-

wavelength: IR, Visible, UV light

wavelength: IR, Visible, UV light

Sh rt

Sh rt l n th X l n th X r & G mm r & G mm r

Short

Short-wavelength: X wavelength: X-

rays & Gamma

rays & Gamma-

rays

rays

Sunlight 9

Clicker Question Clicker Question

How does the sun’s hot surface emit light?

A.

A. Its hot atoms glow with their characteristic

Its hot atoms glow with their characteristic l lik h i i l lik h i i colors, like those in a neon sign. colors, like those in a neon sign.

B.

B. Its hot atoms escape from its surface and

Its hot atoms escape from its surface and become light waves. become light waves.

C.

C. Thermal energy causes its charged particles to

Thermal energy causes its charged particles to accelerate rapidly and thus emit EM waves. accelerate rapidly and thus emit EM waves.

Sunlight 10

Spectrum of Sunlight Spectrum of Sunlight

Sunlight is thermal radiation

Sunlight is thermal radiation— —heat from the sun heat from the sun

Charges in the sun’s hot photosphere jitter thermally

Accelerating charges emits electromagnetic waves

Th

n mit bl k Th n mit bl k b d p tr m t 5800 K b d p tr m t 5800 K

The sun emits a black

The sun emits a black-

body spectrum at 5800 K

body spectrum at 5800 K

We perceive thermal light at 5800 K as “white”

Sunlight 11

Clicker Question Clicker Question

If you compare a sheet of white paper that’s

If you compare a sheet of white paper that’s illuminated by direct sunlight to an identical illuminated by direct sunlight to an identical sheet illuminated by an incandescent sheet illuminated by an incandescent lightbulb lightbulb, ,

A.

A. both sheets will appear equally white.

both sheets will appear equally white.

B.

B. the sunlit sheet will appear more blue.

the sunlit sheet will appear more blue.

C.

C. the sunlit sheet will appear more green.

the sunlit sheet will appear more green.

D.

D. the sunlit sheet will appear more red.

the sunlit sheet will appear more red.

Sunlight 12

Question 2 Question 2

Why does the sky appear blue?

Why is the sun reddish at sunrise and sunset?

SLIDE 3

3

Sunlight 13

Clicker Question Clicker Question

When a light wave encounters an atom, its

When a light wave encounters an atom, its electric field polarizes that atom slightly electric field polarizes that atom slightly h b b d i h h b b d i h

A.

A. so the atom can absorb and reemit the wave.

so the atom can absorb and reemit the wave.

B.

B. but the atom cannot absorb the wave.

but the atom cannot absorb the wave.

C.

C. but the atom cannot emit a wave.

but the atom cannot emit a wave.

Sunlight 14

Rayleigh Scattering Rayleigh Scattering

Rayleigh scattering occurs when

passing sunlight electrically polarizes tiny particles in the air.

That alternating polarization

That alternating polarization acts as a source of light waves acts as a source of light waves, ,

so air particles scatter light

so air particles scatter light— —they absorb and then reemit it. they absorb and then reemit it.

Air particles are too small to be good antennas for light,

so long

so long-

wavelengths (reds) scatter poorly

wavelengths (reds) scatter poorly

while shorter

while shorter-

wavelengths (violets) scatter better.

wavelengths (violets) scatter better.

Rayleigh scattered sunlight is bluish in appearance

The missing blue light reddens the solar disk itself

particularly at sunrise and sunset

Sunlight 15

Introductory Question Introductory Question (revisited)

(revisited)

When you look up at the sky during the day, is

When you look up at the sky during the day, is the light from distant stars reaching your eyes? the light from distant stars reaching your eyes? Y

A. A.

Yes Yes

B. B.

No No

Sunlight 16

Question 3 Question 3

How does a rainbow break sunlight into colors?

Sunlight 17

Light and Refraction Light and Refraction

Sunlight slows while it passes through matter

Light waves electrically polarize matter

That polarization delays the light wave’s passage

E

h m t ri l h n “ind f r fr ti n” E h m t ri l h n “ind f r fr ti n” th f t r th f t r

Each material has an “index of refraction”

Each material has an “index of refraction”—the factor the factor by which it reduces light’s speed. by which it reduces light’s speed.

When light changes speed at an interface,

it bends toward the perpendicular if it slows down

it bends away from the perpendicular if it speeds up

Sunlight 18

Clicker Question Clicker Question

Glass is much more

Glass is much more polarizable polarizable than air. When a than air. When a light wave moves from air into glass, the sudden light wave moves from air into glass, the sudden increase in polarization causes increase in polarization causes

A.

A. part of the wave to be reflected.

part of the wave to be reflected.

B.

B. part of the wave to be absorbed.

part of the wave to be absorbed.

C.

C. all of the wave to be transmitted.

all of the wave to be transmitted.

D.

D. some of the wave to travel along the surface.

some of the wave to travel along the surface.

SLIDE 4

4

Sunlight 19

Light and Reflection Light and Reflection

Light polarizes different materials differently,

which affects both how fast light travels in them and

relationship between its electric & magnetic fields.

Ch i h li h l fl i Ch i h li h l fl i

Changes in how light travels causes reflections

Sunlight partially reflects from most surfaces

Sunlight reflects almost completely from metals

Sunlight 20

Light and Dispersion Light and Dispersion

The different colors of light in sunlight

have different frequencies

and polarize a material slightly differently,

th

tr l t li htl diff r nt p d th tr l t li htl diff r nt p d

so they travel at slightly different speeds.

Violet light usually travels slower than red

Refraction (bending) depends speed change

so violet light usually bends more than red

Sunlight 21

Rainbows Rainbows

Occur when sunlight encounters water droplets

Occur when sunlight encounters water droplets

and undergoes refraction, reflection, and dispersion.

Sunlight 22

Clicker Question Clicker Question

Faceted diamonds sparkle with color when

Faceted diamonds sparkle with color when illuminated by sunlight. Most imitations can’t illuminated by sunlight. Most imitations can’t match this color because light’s speed in them match this color because light’s speed in them

is faster than in diamond.

is slower in them than in diamond.

is less dependent on frequency than in diamond.

is more dependent on frequency than in diamond.

Sunlight 23

Question 4 Question 4

Why are soap bubbles and oil films so colorful?

Sunlight 24

Light and Interference Light and Interference

Overlapping waves superpose and may interfere

Light following different paths can interfere

constructively if fields point in the same direction

constructively if fields point in the same direction d i l if fi ld i i i di i d i l if fi ld i i i di i

r destructively if fields point in opposite directions.
r destructively if fields point in opposite directions.

The two reflections from

The two reflections from a soap or oil film interfere a soap or oil film interfere

Different colors often

Different colors often interfere differently interfere differently

SLIDE 5

5

Sunlight 25

Clicker Question Clicker Question

Sunlight consists of countless independent light

waves. What waves are interfering to produce
waves. What waves are interfering to produce

the colors of a soap film? the colors of a soap film?

A.

A. Parts of the same light wave are interfering.

Parts of the same light wave are interfering.

B.

B. Different light waves are interfering.

Different light waves are interfering.

Sunlight 26

Question 5 Question 5

Why do polarizing sunglasses reduce glare?

Sunlight 27

Clicker Question Clicker Question

When light hits a transparent surface at a

When light hits a transparent surface at a shallow angle, it polarizes the surface. That shallow angle, it polarizes the surface. That polarization is strongest when the light’s electric polarization is strongest when the light’s electric field points field points field points field points

A.

A. along (parallel to) the surface.

along (parallel to) the surface.

B.

B. at right angles to (perpendicular to) the surface.

at right angles to (perpendicular to) the surface.

Sunlight 28

Reflection of Polarized Light Reflection of Polarized Light

Angled reflections depend on polarization

When light’s electric field is parallel to a surface

there is a large fluctuating surface polarization

there is a large fluctuating surface polarization d h fl i d h fl i

and thus a strong reflection.

When electric field is perpendicular to a surface

there is a small fluctuating surface polarization

and thus a weak reflection.

Glare is mostly polarized parallel to the surface

Sunlight 29

Polarization and Sunlight Polarization and Sunlight

Polarizing sunglasses

block horizontally polarized light

and thus block glare from horizontal surfaces.

R l i h i h l i i ff R l i h i h l i i ff

Rayleigh scattering has polarizing effects,

so much of the blue sky is polarized light, too.

Sunlight 30

Summary about Sunlight Summary about Sunlight

Sunlight is thermal light at about

Sunlight is thermal light at about 5800 K 5800 K

It undergoes Rayleigh scattering in the air

It bends and reflects from raindrops

It interferes colorfully in soap and oil films

It reflects in a polarizing fashion from surfaces